Lactation is a reproductive process which is unique to mammalian species and essential for their survival. Although not absolutely critical for human survival in the modem world, breast milk is still considered the optimal nourishment for newborns. However, we still know relatively little about the complex neural circuitry in the central nervous system, which contributes to lactational processes. The endogenous opioid peptides influence hormone release and behaviors that are unique to lactation. However, there are multiple opioid peptides and opioid receptor subtypes localized in specific brain areas. Each neuronal group appears to have a distinct function(s) and be differentially regulated. The precise role(s) and regulation of each group during lactation has not been clearly defined. The endogenous opioid peptides are an important component in the neuronal network to translate the suckling stimulus into increased prolactin (PRL) secretion in some species. PRL is an anterior pituitary hormone which is fundamental to maintain adequate lactation. As with many pituitary hormones, PRL regulates its release by a short-loop feedback mechanism to hypothalamic neurons, but it is not entirely clear at what levels this feedback loop is operational in lactating animals. Indeed, PRL feedback to dopaminergic neurons in the hypothalamus is diminished during lactation. The primary goal of this proposal is to evaluate the expression of the endogenous opioid peptide precursors in the hypothalamus during lactation. The first specific aim will explore the regulation of the opioid peptide gene expression in specific hypothalamic nuclei under conditions that alter the lactational state. The effect of pup deprivation on mRNA signal levels for proenkephalin, prodynorphin and proopiomelanocortin in the hypothalamus will be examined by in situ hybridization histochemistry. The second specific aim will evaluate opioid peptide mRNA signal levels after hormonal manipulations in lactating dams. In particular, the role of PRL in regulating the opioidergic neurons during lactation will be examined. Overall these studies will contribute to our understanding of the complex neuronal pathways involved in maintaining lactation.